Japanese patent No. 2764495 shows a conventional knocking detecting device for an automobile. In this device, an analog signal output from a knock sensor is analog-to-digital converted in a constant sampling period and the converted data (sampling data) on the time series are digital filter processed. Then, a knocking is detected base on the sampling data. An analog-to-digital conversion is referred as A/D conversion and an analog-to-digital converter is referred as A/D converter herein after.
A knocking detection is processed according to all filter-processed data which are saved every sampling timing during a knocking detection period. During knocking detection, a knock sensor signal is processed and the crankshaft of an internal combustion engine rotates in a predetermined angle.
Since the device takes a long time to detect the knocking in low rotation of an engine, a large number of the data should be stored so that a memory capacity and processing load are increased.
To avoid such a problem, it is suggested that a filter-processed data are integrated to every fixed number N and the existence of the knocking is detected from the integrated value to reduce the stored data. In such a process, the integrated filter processed data can be deleted and only integrated data are stored so that which the number of the stored data can be reduced to “1/K”. As shown in FIG. 13, the knocking detection is carried out according to the fluctuation of the integrated value.
The device described above needs a long period to detect knocking and the number of stored data (the number of integrated number) is increased in a low rotation of the engine. If the value N is increased to decrease the number of integrated value, the number of integrated value is reduced in a high rotation of the engine and the knocking detection cannot be carried out precisely. That is, if the number of the integrated value is too small, the characteristic of the knocking cannot be detected.
To avoid such a problem, it suggested that A/D converted and digital-filtered data are integrated every period in which a crankshaft rotates to predetermined angle R (for example 5° CA) instead of every fixed number. That is, a timing signal arises every timing in which the crankshaft rotates to a predetermined angle R, the filter-processed data are integrated during a time period from the timing signal arising to the next timing signal arising. Consequently the number of the integrated value is restrained to a proper number to detect the knocking. “CA” is referred to a rotational angle of the crankshaft (crank angle).
In the case of integration of the filter-processed data every period in which a crankshaft rotates to predetermined angle, another problem arises.
The filter-processed data every sampling timing is grouped every time period of crank angle. The number of the filter-processed data which are integrated in a predetermined angle (5° CA) fluctuates as shown FIG. 14A. Dots in FIG. 14 represent a sampling timing (A/D conversion timing) of the knock sensor signal and a timing of calculation of new filter processed data.
When 5° CA indicates 85 μs and a sampling period of the knock sensor signal indicates 20 μs (50 k Hz), four or five data are integrated are integrated in the 5° CA period. The number of the filter-processed data in an integrated value fluctuates.
As shown in FIGS. 14B and 14C, the number of filter processed data fluctuates due to an acceleration or deceleration of the engine. “NE” in FIGS. 14B and 14C represents a timing signal which traverses every 5° CA.
When the number of the filter-processed data in each integrated value fluctuate, the integrated values also fluctuates due to the other condition besides the wave of the knock sensor signal. Thus, the knocking cannot be detected precisely and the feature of the knocking can no be obtained precisely.